TECTONIC STRATIGRAPHY OF THE LARAMIDE DENVER BASIN FORELAND: INSIGHTS FROM DETRITAL ZIRCON U-PB GEOCHRONOLOGY

Detrital zircon U-Pb ages from the latest Cretaceous-Eocene fill of the Denver Basin provide new insights into evolving sediment dispersal patterns during filling of an intra-continental foreland basin. A suite of sandstone samples from the Castle Pines and Kiowa drill cores provide vertical transects through pre- and syn-orogenic Denver Basin fill in proximal and distal portions of the basin, respectively. Preliminary data from fourteen sandstone samples from the Kiowa drill core reveal that pre-orogenic samples (Pierre Shale and Fox Hills Sandstone) of latest Cretaceous age contain abundant 1.65-1.8 Ga and ca. 1.4 Ga zircon, with additional age populations spanning Late Cretaceous through Archean time. These grain ages are consistent with a sediment source area in the western U.S., including Yavapai-Mazatzal basement and the Mesozoic Cordilleran arc, with lesser contributions of recycled Grenville and Appalachian zircon from older sedimentary sequences. Seven of the eleven samples from the syn-orogenic fill of the Denver Basin display a pronounced 1.05-1.15 Ga peak that likely reflects derivation from the local 1.08 Pikes Peak granite, thus recording a major provenance shift during uplift and denudation of the southern Front Range. However, four of the syn-orogenic samples yielded detrital zircon age populations similar to pre-orogenic units, with major age peaks at ca. 1.7 and 1.4 Ga and lacking the 1.1 Ga age peak found in the other syn-orogenic samples. Three of these anomalous samples were collected from a fine-grained, coal-bearing zone in the middle of the D1 Sequence (latest Cretaceous-Paleocene). We hypothesize that these samples represent the interplay between local transverse and extra-regional longitudinal sediment dispersal systems that can be distinguished on the basis of detrital zircon U-Pb age distributions. This preliminary interpretation departs from previous studies that have largely interpreted the syn-orogenic fill of the basin to have been supplied by transverse rivers emanating from the southern Front Range. This study also has implications for the unroofing history of the southern Front Ranges and for evolving sediment transport pathways during the Laramide Orogeny.